Idiopathic pulmonary fibrosis (IPF) is a progressive scarring disorder of the lung, and patients suffering from this condition experience significant morbidity and mortality. Unfortunately, treatment options for these individuals are limited, and no intervention has significantly impacted the severe disease course. As such, we are in desperate need of new candidate therapies. Plasminogen activator inhibitor-1 (PAI-1) provides a promising target for the treatment of IPF based on observations that intrapulmonary levels of PAI-1 are consistently elevated in patients with fibrotic lung disease and that the level of PAI-1 tightly correlates with the severity of fibrosis in several different animal models of lung scarring. Although the mechanism by which PAI-1 promotes lung fibrosis remains unclear, we made significant advances in our understanding of its function during our previous funding period. First, we demonstrated that PAI-1 drives fibrogenesis through its somatomedin B binding domain rather than its more well-recognized antiprotease function. Second, we have identified a role for PAI-1, through its SMB-binding activity, in the intra- pulmonary accumulation of profibrotic Ly6Chigh monocytes and their derivative exudate macrophages following lung injury. Despite these advances in our understanding of PAI-1's role in pulmonary fibrosis, there remain many unanswered questions. First, the target protein for PAI-1's somatomedin B binding domain has not been identified. The most well defined binding partner for PAI-1 is vitronectin, making this matrix molecule a potential candidate. However, our preliminary data indicate that PAI-1 interacts with a yet-to-be described binding partner to exert its pro-fibrotic effect. Second the mechanism by which PAI-1 regulates monocyte/macrophage lung accrual following injury has yet to be determined. And third, the requirement for PAI-1 in monocyte/macrophage accumulation following lung injury raises the question as to whether a third PAI-1 function, its interaction with the low density lipoprotein-like receptor (LRP), contributes to fibrogenesis as ths activity has been shown to facilitate macrophage motility. The aforementioned knowledge gaps serve as the motivation for the specific aims of this proposal. Specifically, we plan: 1) To demonstrate the molecular interactions by which the SMB-binding function of PAI-1 exerts its pro-fibrotic activity, 2) To determine the mechanistic link between PAI-1, monocyte/macrophage accrual, and lung fibrosis., and 3) To define the contribution of the LRP-interactive activity of PAI-1 in pulmonary fibrosis. Because PAI-1 has such a potent effect on the severity of fibrosis in multiple different animal models, it is certain to play a central role in disease pathogenesis. As such, completion of these aims will provide core insights into the pathobiology of lung fibrosis and refine therapeutic strategies for patients with IPF.